As shown in FIG. 1, a PON communication system comprises the following parts: an optical line terminal (OLT) C01, a trunk optical fiber C02, an optical splitter network C03, an optical network unit (ONU) C04 and a branch optical fiber C05. The trunk optical fiber C02, the optical splitter network C03 and the branch optical fiber C05 are collectively referred to an optical distribution network (ODN) which is a passive system, therefore, any network with this kind of architecture is named as passive optical network (PON).
PON communication system supports multi-service transmission and needs to provide reliability at a telecommunication level, and protection switching is its important content. ITU G.984 series and G.983 series define four protection methods of a PON system. The system structure shown in FIG. 2 only targets for optical fiber backup and only supports cold backup; the system structure shown in FIG. 3 only targets for OLT backup and only supports cold backup; what is shown in FIG. 5 is the system structure for half-backup, providing end-to-end protection in cold backup, as well as the trunk optical fiber and the OPT interface circuit protection in cold backup. The protection types offered in the above three system structures are all cold backups and need new distance measurement and registration after the active and backup systems perform protection switching, and its protection switching speed can not meet the telecommunication level requirement of 50 ms.
FIG. 4 shows a system structure of full backup, providing end-to-end protection of the ONU, the interface circuits of OLT and the optical fiber links (including trunk fiber and branch fiber) between them. This type of protection is hot backup and does not need new distance measurement and registration after protection switching, therefore it is able to offer 50 ms protection switching at telecommunication level. The standard G.983 specifically describes and regulates the protection mechanism in FIG. 4.
Everything in the full backup system in FIG. 4 is doubled: two OLT interface circuits, two trunk fibers, two optical splitters, two branch fibers, and the ONU with two interface circuits, therefore the cost is high and it is only suitable for the optical fiber-to-resident corporation users or enterprise users with high demand for reliability, and it is not suitable for public users who are more sensitive with price. In this system, if only one branch optical fiber is set between the optical splitter and the ONU, such as the connection to ONU 1 in the figure, then there will be no protection for the users of this unit.
When the PON is applied in the optical fiber-to-the-home (FTTH) project for public user, the damage of one branch fiber only impacts the single user that is connected with that branch fiber while no one else will be affected, however, when the trunk fiber or OLT is damaged, the entire PON will be paralyzed. Therefore, considering feasibility, reliability and performance, when PON is used for broad public users, it needs to provide the protection time of 50 ms, but branch optical fiber belongs to single users and no protection is needed. Because the optical splitter in the public parts is passive and is placed in the boxes and small rooms with guard, the reliability is high and no backup is needed, in other words, the protection part is only designed for trunk fiber and OLT in public parts. There are only two protection types offered in FIG. 4, either full-link end-to-end protection, or no protection at all, which can not meet the protection demand of public users.